1 Micromechanics of macroelectronics Zhigang Suo Harvard University Work with Teng Li, Yong Xiang, Joost Vlassak (Harvard University) Sigurd Wagner, Stephanie Lacour (Princeton University)

2 Displays Sony e-Reader

3 Roll-to-roll printing Low cost, large area

4 polymer inorganic defect Challenges to the mechanics of materials and structures Large structures Hybrid materials (organic/inorganic) Small features Polymer substrate Active device Hermetic seal Thin-film transistor (TFT) Al undoped a-Si:H SiN x Ti/Cr 100 nm 360 nm 100 nm 180 nm SiN x (n + ) a-Si:H 50 nm

5 How to make brittle materials flexible? Thin substrate Strain caused by bending Small flaws Strain to cause fracture R Neutral plane c  top Suo, Ma, Gleskova, Wagner Appl. Phys. Lett. 74, (1999).

6 G S D TFT island a-Si thin-film transistor (TFT) on polyimide substrate High strain and fracture Gleskova, Wagner, Suo Applied Physics Letters, 75, 3011 (1999) Cracks

7 How to make stretchable circuits? Islands, linked by interconnects Fracture at crossovers Fatigue of metals Small island size Most microelectronic materials fracture at small strains (less than about 1%) Polymer substrate Springs 3D microfabrication Hsu, Bhattacharya, Gleskova, Huang, Xi, Suo, Wagner, Sturm, APL 81, 1723 (2002).

8 G S D TFT island a-Si thin-film transistor (TFT) on Kapton substrate Debonding and cracking Gleskova, Wagner, Suo Applied Physics Letters, 75, 3011 (1999) Cracks SiN island on Kapton substrate Bhattacharya, Salomon, Wagner J. Electrochm. Soc. 153, G259 (2006)

9 Metal on polymer Al, Cu, Au Metal film deforms plastically (Ho, Kraft, Arzt, Spaepen…) What is the rupture strain of the metal film? Kapton, Silicone

10 Ductile vs. brittle film metal film  Rupture by necking ceramic film  Rupture by breaking atomic bonds ceramic film polymer substrate metal film polymer substrate

11 FEM: large-amplitude perturbation Free-standing Substrate-bonded Long-wave perturbation Conclusion from nonlinear analysis: Substrate retards perturbation of ALL wavelengths. Substrate-bonded short-wave purturbation  =0.8 Li, Huang, Suo, Lacour, Wagner, Mechanics of Materials 37, 261 (2005)

12 Al film on Kepton substrate 5000 Å Al film, 7 % Strain5000 Å Al film, 10 % Strain Gage, Phanitsiri (2001) Chiu, Leu, Ho, (1994) Alaca, Saif, Sehitoglu (2002) Channel cracks start at ~2% strain

13 Possible causes for small rupture strains of metal on polymer The film is brittle. The film debonds from the substrate. The substrate is too compliant.

14 Co-evolution: necking and debonding Coupled rupture and debond EE10.4 Thursday 2:30pm, Teng Li Ductility of thin metal films on polymer substrates modulated by interfacial adhesion.

15 T 22 35% 37.3% 38% 38.5% T 12 35% 37.3% 38% 38.5% Li, Suo, IJSS (2006)

16 Xiang, Li, Suo, Vlassak, APL 87, (2005) 100nm Cu /10nm Ti/Kapton, strained to 10% 100nm Cu /20nm C/ Kapton, strained to 6% Effect of adhesion

17 170nm Cu /10nm Ti/ Kapton strained to 30% Xiang, Li, Suo, Vlassak, APL 87, (2005)

18 The effect of substrate stiffness E sub = 2 MPa,  = 2.8% E sub = 300 MPa,  = 47% E sub = 150 MPa,  = 37% Li, Huang, Suo, Lacour, Wagner, Appl. Phys. Lett. 85, 3435 (2004)

19 Au film on PDMS substrate survives large elongation electron-beam evaporation 101 cycles of elongation by 35% PDMS (1mm) Cr (5 nm) Au (25 nm) Lacour, Wagner, Huang, Suo,, APL 82, 2404 (2003).

20 Au film is cracked from the beginning, but… 1µm stretching direction b 1µm stretching direction c 1st cycle to 35% strain 1µm a As deposited 101st cycle to 35% strain Lacour, Li, Chen, Wagner, Suo, APL 88, (2006).

21 Other Compliant Patterns When pulled, the sheet elongates by buckling Y-shaped cracks

22 The importance of being compliant

23 Serpentine: a compliant pattern of a stiff material Top surface Bottom surface Li, Suo, Lacour, Wagner, JMR 20, 3274 (2005) Large elongation, small strain A platform for devices

24 Summary A stiff polymer substrate can retard necking in a metal film. A compliant polymer substrate can accommodate large displacement of a patterned film.